The present invention relates to a thermal printer, and more specifically, to a thermal printer in which the printing conditions can be changed in accordance with a kind of a ink ribbon used.
In general, a thermal printer is equipped with a thermal head provided with a plurality of heating elements arranged in a vertical single column, and printing is carried out in such a manner that a carriage mounted on the thermal head is moved with respect to a recording sheet in a direction perpendicular to the direction in which the heating elements are arranged while the thermal head is pressed against the recording sheet and at the same time, a pulse voltage is applied to all of the heating elements or selectively applied to a part thereof to print characters and the like in a dot pattern on a thermosensitive recording sheet or a usual recording sheet through a ink ribbon.
Conventionally, one-time ink ribbons are widely used, but correctable ink ribbons have been produced to enable incorrectly typed characters to be instantly corrected when printing is carried out while characters are input through a keyboard of an electronic typewriter provided with a thermal printer, and more recently, multi-time ink ribbons have been produced to permit ink ribbons to be used several times, to enable a reduction in the cost of using ink ribbons.
As shown in FIG. 1, a one-time ink ribbon 49 has a four-layer structure composed of a top coat layer 49a having a high melting viscosity and relatively poor adhesion to a recording sheet, an ink layer 49b on which resin type transfer ink is coated, a base film 49c such as polyester or the like, and a sticking prevention layer 49d composed of a heat resistant resin.
As shown in FIG. 2, a correctable ink ribbon 50 (refer to Japanese Provisional Patent Publication Sho 62-108090) has a five-layer structure composed of a top coat layer 50aan ink layer 50b on which resin type transfer ink is coated, an exfoliation layer 50c composed of wax and having a poor adhesion with a base film 50d, the base film 50d, and a sticking prevention layer 50e.
As shown in FIGS. 3 (a), (b), and (c), when printing is carried out, the correctable ink ribbon 50 is pressed against a recording sheet 58 by a thermal head, and a voltage is applied to heating elements 22. The correctable ink ribbon 50 is heated through the sticking prevention layer 50e, and thus, as shown in FIG. 3 (a), the exfoliation layer 50c is melted so that the top coat layer 50a is adhered to the recording sheet 58 as a transfer ink 80 together with the ink layer 50b, by the adhesive force of the top coat layer 50a.
On the other hand, as shown in FIG. 3 (b), when an incorrectly printed character must be deleted, the correctable ink ribbon 50 is overlapped on and pressed against the character and the heating elements 22 are heated by the application of a pulse voltage. In this case, the exfoliation layer 50c, the ink layer 50b, the top coat layer 50a, and the transfer ink 80 are melted, respectively, through the sticking prevention layer 50e and the base film 50d, and then the exfoliation layer 50c, the ink layer 50b, the top coat layer 50a, and the transfer ink 80 are cooled, respectively, in a predetermined time after the supply of voltage to the heating elements 22, whereby the transfer ink 80 and the top coat layer 50a are firmly adhered to each other for solidification. Since the adhesion between the recording sheet 58 and the transfer ink 80 is weaker than the firm adhesion between the respective five layers 5a to 5e and the firm adhesion between the top coat layer 5a and the transfer ink 80 in this case, when the correctable ink ribbon 50 is separated from the recording sheet 58, the transfer ink 50 remains adhered to the ink ribbon 50 and thus removed from the recording sheet 58, as shown in FIG. 3 (c).
As shown in FIG. 4, a multi-time ink ribbon 51 (refer to Japanese Provisional Patent Publication Sho 61-68290) has a three-layer structure composed of a wax type ink layer 51a or the like, a base film 51b, and a sticking prevention layer 51c. Further, the ink layer 51a is provided with a so-called "stone wall" structure to enable characters to be printed by the multi-time ink ribbon 51 several times.
More specifically, since each of these one-time ink ribbon 49, correctable ink ribbon 50, and multi-time ink ribbon 51 has a specific ribbon structure, the kind and melting temperature of the ink and the transferability thereof are slightly different. Consequently, printing conditions such as a thermal head pressing force, a voltage applied to heating elements, an energizing time thereof, a ribbon exfoliating angle, and a printing speed must be individually set in accordance with the kind of ink ribbon used to effect printing, to obtain an optimum printing efficiency.
When a voltage is applied to the heating elements, the ink is melted and transferred onto a recording sheet. After predetermined time has passed, the heating elements are turned off and the melted ink begins to be solidified. In this case, as the carriage is being moved, the ink ribbon is left-adhered on the recording sheet without being depressed. As each layers of the ink ribbon has a different solidification time and a different adhesive characteristic, the interval between the beginning of solidification of the melted ink and exfoliation of the ink ribbon from the recording sheet affects the quality of printed or corrected character images. For example, in case of using the correctable ink ribbon 50 (FIG. 3), the ribbon 50 should be exfoliated after such time has passed that the transferred ink 80 on the recording sheet is left therefrom and sufficiently adhered onto the ribbon 50. This time interval is determined in accordance with the following two factors, that is, the speed of the carriage moved in the width direction of the recording sheet and the exfoliating angle formed by the recording sheet and the ink ribbon at the portion where the ink ribbon is exfoliated from the recording sheet. Assuming that the winding force of the ink ribbon is constant, the larger the exfoliating angle is, the larger the force vector is in the direction that the ribbon is away from the recording sheet. Further, the large the force vector in the direction that the ink ribbon is away from the recording sheet is, the easier it becomes to exfoliate the ink ribbon from the recording sheet against the adhesive force of the ink ribbon. Therefore, the time interval between beginning of solidification of the ink and exfoliation of the ink ribbon becomes short in this case. On the other hand, the smaller the exfoliating angle is, the smaller the force vector is, and the time interval becomes long.
Since the one-time ink ribbon 49 is used only for printing, preferably the ribbon exfoliating angle and pressing force are relatively large, and the ink is instantaneously melted by the application of a high voltage to ensure a sufficient transfer thereof to a sheet for printing. Further, although the multi-time ink ribbon 51 is also used only for printing, preferably the printing is carried out with a relatively small ribbon exfoliating angle and pressing force, so that the ink of the ink layer 51 can be used for more than one printing.
On the other hand, when printing with the correctable ink ribbon 50, by which a character may be deleted after it has been once printed, the printing conditions are preferably set in such a way that the stripping of ink transferred when an incorrect character has been printed on a sheet is improved.
Since, however, a conventional thermal printer assembled in an electronic typewriter or the like is not able to arbitrarily change the above printing conditions, the kind of ink ribbons able to be used for printing is specified for each type of machine.
Recently, a thermal printer by which a ink ribbon exfoliating angle of these printing conditions can be changed has been proposed. For example, Japanese Provisional Patent Publication Sho 62-30074 discloses a thermal printer wherein a lever is provided with guide pins for guiding a passage of a ink ribbon, and a thermal head is rotatably disposed at the rear edge of a carriage, the lever is turned in accordance with the flatness of a recording sheet, and thus the guide pins are moved to change the ink ribbon striping angle.
Since the above thermal printer disclosed in Japanese Provisional Patent Publication Sho 62-30074 is able to change only the ribbon exfoliating angle, when a ink ribbon to be used is changed., a pressing angle, a voltage applied to heating elements and an energizing time thereof cannot be changed, even though the ribbon exfoliating angle can be changed, and thus a problem arises in that a desired ink ribbon cannot be used in accordance with a printing object.
Since conventional thermal printers cannot change various printing conditions such as a ribbon exfoliating angle, pressing force and the like, when a ribbon other than a dedicated ribbon is used, the printing efficiency is greatly lowered and sometimes printing cannot be carried out. More specifically, a problem arises in that a ink ribbon cannot be arbitrarily selected in accordance with a printing object, and when printing is carried out using many kinds of ink ribbons, the thermal printer must be prepared in accordance with the kind of ribbon used and the like.